The present invention relates to electro-optical devices and more particularly to improved liquid crystal displays.
Conventional liquid crystal displays utilize two spaced plane-parallel glass plates with conductive electrode patterns and a layer of liquid crystal material confined therebetween. Traditionally, polarizers are placed externally to the glass plates. Light entering one side of the display is polarized along one axis, altered as it passes through and as a function of the liquid crystal material, and exits through the other polarizer, typically in an orthogonal relation to the first polarizer. Application of a voltage potential across selected electrodes alters the orientation of the liquid crystal molecules therebetween to locally alter the optical characteristics of the cell and to achieve the desired optical display effect. More sophisticated displays may incorporate a reflector or a transreflector to provide a display that can operate in either a light reflecting or light transmitting mode.
In the manufacture of liquid crystal display devices, the polarizers are typically located externally to the glass plates so that each glass plate is positioned between a polarizer and the liquid crystal layer. Since the glass is optically isotropic, the glass does not adversely affect the polarized light dynamics. Glass plates are well suited for liquid crystal display applications since they are optically transparent and isotropic, rigid, dimensionally stable and impervious to gases. The use of glass plates, however, poses a number of drawbacks with regard to cost and ease of manufacturing since glass is comparatively fragile and not well suited to high-speed automatic fabrication techniques.
It has been suggested, for example, in U.S. Pat. No. 4,228,574 to Culley et al. to replace the glass plates with plastic. However, the substitution of plastic materials for the glass can lead to problems that affect cell performance. For example, polyester plates, while transparent and chemically compatible with liquid crystal materials, are typically biaxially oriented and birefringent. These birefringent plates, owing to their light-depolarizing properties and positioning intermediate the externally placed polarizers, serve to adversely affect cell performance.
Stretching polyester to provide a uniaxially oriented material to solve the problems associated with biaxial polyester gives rise to other problems. For example, it is preferred from the standpoint of dimensional stability, particularly as affected by temperature variations, to utilize each of the uniaxial support sheet materials in parallel orientation. In addition, it is preferred to utilize each of a pair of orthogonal polarizers of a liquid crystal display device at an angle of 45.degree. with respect to the vertical and horizontal dimensions of the display. The requirement that each of the uniaxially oriented polyester plates be aligned in parallel relation to its adjacent polarizer, or perpendicularly with respect to such polarizer, dictates that the polyester plates be cut diagonally from an uniaxially stretched polyester web and that they be properly aligned with the externally placed polarizers. This process complicates the manufacturing process and generates considerable scrap material. In addition, the uniaxially oriented polyester material may exhibit an undesired amount of birefringence.
Other materials that are optically transparent and optically isotropic, such as cellulose acetate butyrate, may require modification with plasticizers in order to obtain essential physical properties. These modifications result in materials that are chemically incompatible with known liquid crystal materials and mixtures.
It has also been suggested in U.S. Pat. No. 4,241,984 to Leibowitz, to place the polarizers internally of the transparent glass plates, over the conductive electrodes and in contact with the liquid crystal material, so that the surface characteristics of orthogonally oriented polarizers also effect the desired twisted alignment of the nematic liquid crystal material. Polarizing materials, however, may be chemically incompatible with the liquid crystal materials and the thickness of the polarizer necessary for reasonable optical efficiency would be inconsistent with the need to achieve the necessary voltage gradient across the liquid crystal material at moderate supply voltage.